Evaluation of the Mechanical and Thermal Properties Decay of PHBV/Sisal and PLA/Sisal Biocomposites at Different Recycle Steps.

The recyclability of polylactide acid (PLA) and poly (3-hydroxybutyrate-co-3-hydroxyvalerate (PHBV)-based biocomposites (10%, 20% and 30% by weight of sisal natural fibre) was evaluated in this work. The mechanical and thermal properties were initially determined and were shown to be similar to commodity plastics, such as polyethylene or polypropylene. Three recycle steps were carried out and the mechanical and thermal properties of recycled samples were evaluated and compared to the reference samples. The tensile modulus increased for recycled PLA biocomposites, whereas it was hardly influenced by recycling the PHBV biocomposites. The tensile strength and deformation at the break decreased notably after the first cycle in all cases. Although all the biocomposites became more brittle with recycling, the properties were conserved along until the third cycle, proving their promising recyclability. From the data obtained from the dynamic mechanical analysis, a slight decrease of the storage modulus of PHBV was observed, whereas PLA showed a significant decay of its properties at the 3rd recyclate. The PLA specimens were filled with sisal fibres until they reached 20%wt, which seemed also less subject to the embrittlement occurring along the recycling phase. The characteristic temperatures (glass transition-Tg, crystallization-Tc, melting-Tm) of all the biocomposites were not highly affected by recycling. Only a slight decrease on the melting point of the recycled PHBV was observed suggesting an overall good reprocessability. Moreover, the processing conditions lied in the same range as the conventional plastics which would facilitate potential joint valorization techniques.

Effect of vacuum and of strong adsorbed water films on micropore formation in aluminum hydroxide xerogel powders.

Aluminum hydroxide gels were washed with water, ethanol, methanol and isopropanol to obtain new gels with different liquid phases that were dried either in air at 120 degrees C or under vacuum at 80 degrees C. Drying in air leads to alcoholic xerogels with BET surface areas larger than the aqueous ones. The effect of the alcoholic groups as substitutes of the hydroxyl ones has been discussed to account for the final size of xerogel crystallites. Drying under vacuum decreases the BET surface of the methanol xerogels, but no micropores are formed in all the alcoholic xerogel matrixes. On the contrary, the vacuum drying process changes significantly the microstructure of the aqueous xerogels. Their BET surface increases by 34 m(2)/g, and micropores are formed within their crystallite aggregates. It has been experimentally shown that these changes are due to a shear transformation that occurs in the boehmite xerogels obtained under vacuum. To discuss these data, the existence of chemical compounds such as AlOOHnH(2)O was postulated. On this ground, a neat analogy between vacuum drying process and vacuum interfacial decomposition reactions of inorganic salts can be drawn. This analogy explains how a state of stresses forms in aqueous xerogel matrix during vacuum drying process.

Aluminum hydroxide microstructural units in gelled media aged, or nonaged, with alcohol and water.

Aluminum hydroxide gelled media dispersed and then aged in alcohol (ES) or in water (WS) were prepared with a solid phase concentration equal to 0.45% v/v. The media were characterized through rheological measurements and detailed analysis. Xerogels were obtained through a designed drying process at 8 degrees C under vacuum for 24 h, followed by a heating treatment at 105 degrees C and a pressure of 10(-3) Pa for 2 h. The xerogel microstructure was carefully investigated using N2 adsorption-desorption methods at 78 K. Rheology data on ES and WS colloidal dispersions were compared with data computed from adsorption isotherms of corresponding xerogels, in order to infer information about particles organization in the dispersions. On this ground, a model for microstructural units dispersed in the liquid phase has been derived. The beneficial action of alcohol in giving xerogel powders with higher specific surface area and higher porosity has been explained on the basis that alcohol produces changes in the corresponding dispersed microstructural units. The effect of the aging time in alcohol and in water was evaluated, and the evolution of all microstructural parameters was described.

A bone substitute composed of polymethylmethacrylate and alpha-tricalcium phosphate: results in terms of osteoblast function and bone tissue formation.

The biological properties of a composite polymeric matrix (PMMA + alpha-TCP) made of polymethylmethacrylate (PMMA) and alfa-tricalciumphosphate (alpha-TCP) was tested by means of in vitro and in vivo investigations. PMMA was used as a comparative material. Osteoblast cultures (MG 63) demonstrated that PMMA + alpha-TCP significantly and positively affected osteoblast viability, synthetic activity and interleukin-6 level as compared to PMMA. At 12 weeks, the PMMA + alpha-TCP implants in rabbit bone successfully osteointegrated in trabecular and cortical tissue (affinity index: 57.14+/-8.84% and 68.31+/-6.18%, respectively). The newly formed bone after tetracycline labelling was histologically observed inside PMMA + alpha-TCP porosity. The microhardness test at the bone-PMMA + alpha-TCP interface showed a significantly higher rate of newly formed bone mineralization compared with PMMA (+83.5% and +58.5%, respectively), but differences still existed between newly formed and pre-existing normal bone. It is herein hypothesized that the present positive results may be ascribed to the porous macroarchitecture of PMMA + alpha-TCP and the presence of the bioactive ceramic material that could have a synergic effect and be responsible for the improvement of (a) the material colonization by bone cells, (b) osteoblast activity, (c) osteoinduction and osteoconduction processes, (d) bone remodelling.

The effect of water in inorganic microsponges of calcium phosphates on the porosity and permeability of composites made with polymethylmethacrylate.

Bone-substitute compounds were obtained by mixing pre-polymerised powders of polymethylmethacrylate and liquid methylmethacrylate monomer with two aqueous dispersions of alpha-tricalcium phosphate (alpha-TCP) grains and calcium-deficient hydroxyapatite (CDHA) powders. The final composites appeared to be porous. The total open porosity was a function of the amount of water added. The water, which was the pore-forming agent, vapourised after the polymerisation process, leaving behind empty spaces in the polymeric matrix. The inorganic powders placed inside the polymeric matrix were shown to act as local microsponges. The water capacity of these microsponges can be determined by a centrifugation step carried out on aqueous dispersion of alpha-TCP and/or CDHA powders that occur before any reaction with the organic compound. The relationship between the total open porosity of the composites and the amount of water trapped inside the inorganic agglomerates proved to be almost linear. The effect of the chemical composition of the powder on the total open porosity is not too great, provided that the two kinds of pellets are prepared with the same amount of water. Both the permeability and shape of the pores proved to be a function of the total open porosity. An increase of the latter parameter produces an increase in permeability and a decrease in tortuosity. Osteoconductivity and osteoinductivity of the final composites were tested in vivo by implantation in rabbits. Formation of new trabecular bone was observed inside the pores where the inorganic powders had been placed.